By the term mechanical pulps is understood pulps produced primarily by mechanical processing with or without auxiliary steps of chemical or physical nature, such pulps include conventional (stone) ground wood and refiner groundwood and pulps produced by an array of chemi-mechanical and thermo-mechanical processes.
It has been stated for example in U.S. Pat. No. 3,802,964 issued Apr. 9, 1974 to Forgacs and Karnis that the fibre characteristics of average specific surface and particularly of average fibre length are important in determining the properties of a paper produced from said pulp. The average fibre length is very important for predicting tensile, bursting, tearing and wet web strengths and also affects light scattering coefficient (opacity) while the fibre length distribution is important in relation to tear strength since for a given average fibre length generally the wider the distribution the higher the tear strength. However, no on-line methods of determining average (median) fibre length or fibre length distribution are available and such determinations could only be made in a laboratory.
It has been proposed to determine the L-factor (which is defined as the total amount of pulp retained on the 48-mesh screen of a Bauer McNett fibre length Classifier expressed as a percentage by weight of the feed) or weight-average fibre length by the weight ratio of a through or retained fraction to the feed as described in U.S. Pat. No. 3,873,416 issued Mar. 25, 1975 to Forgacs and Karnis, but, as above indicated, notechniques are available for determining, on-line, the fibre length distribution by weight of a mechanical pulp.
Consistency meters currently used in the pulp and paper industry measure consistency indirectly i.e. they measure other properties of the pulp, such as resistance to flow, dielectric properties of the pulp suspension etc. which properties are related to consistency as well as other fibre properties.
It has been proposed to measure consistency substantially independent of other pulp properties using the Sperry Gravity Master (see Management & Control, Vol. 1, Nov. 11, 1968, pp T179-T186) which claims an accuracy for consistency over the range of 0.4 to 1.5% within the range of 12 to 3% of the consistency measured. This instrument operates on the technique of continuously measuring the weight of stock flowing through a specific volume of pipe.
It has also been proposed by Thiessen and Dagg, published in the Pulp & Paper magazine of Canada September 1959 to measure consistency by recording the vibrations resulting from rotation of an unbalanced rotor, the degree of unbalance being determined by the difference in weight between given amount of water with the same amount of pulp slurry.
Neither of the above devices have proved to be commercially satisfactory. No instrument for on-line measurement of pulp consistency substantially free of other characteristics of the pulp is currently known.